1. Field of the Invention
The present invention relates to screen processing.
2. Description of the Related Art
Conventionally, an area gradation method for realizing a gradation reproduction has been used to form an image on a recording media. The area gradation refers to a method for representing gradation by changing the ratio of an area to which a color material adheres. An amplitude modulation (AM) screen and a frequency modulation (FM) screen are representative ones. The AM screen represents a gradation image by modulating the size of an area (the size of a so-called halftone dot) to which a color material adheres and is characterized by the shape of a halftone dot, the direction at which a halftone dot is arranged (a halftone dot angle), and the arrangement density of a periodic halftone dot (the number of lines). On the other hand, the FM screen represents a gradation image by the density of dots by arranging minute isolated dots of a fixed size in a pseudo random manner. The minute dots need to be stably recorded when using the FM screen. In general, an image forming apparatus which is unstable in the reproduction of minute dots uses the AM screen.
In a case where the AM screen is used, so-called moire can occur in an output image. The moire refers to a visible periodic pattern which is generated when an input image interferes with periodically arranged halftone dots and high frequency components in the input image are folded back to a low frequency area. The following two methods are proposed as a method for suppressing such moire.
In a first method, an input image is subjected to filter processing to remove frequency components which may cause moire, from the input image (refer to Japanese Patent Application Laid-Open No. 08-051536). In a second method, if the moire occurs in using the AM screen, the FM screen in which minute isolated dots are arranged in a pseudo random manner is used instead of using the AM screen (refer to Japanese Patent Application Laid-Open No. 2007-129558). However, the techniques discussed in the above documents may cause the following problems.
In the method discussed in Japanese Patent Application Laid-Open No. 08-051536, the input image is subjected to filter processing using a low-pass filter to remove frequency components which causes moire, from the input image. The frequency components causing moire are high frequency components in the vicinity of a screen frequency. If a low pass filter sufficient to remove the frequency components causing moire is applied to the input image, a generally blurred image is produced.
In the method using the FM screen discussed in Japanese Patent Application Laid-Open No. 2007-129558, an image forming apparatus unstable in the reproduction of dots such as a printing device typified by offset printing or an electrophotographic recording apparatus has a problem that an output image is conspicuously rough.